a) Field of the Invention
The present invention relates to a solid-state image pickup device, and more particularly to a solid-state image pickup device of a MOS (metal oxide semiconductor) type having analog/digital converters.
b) Description of the Related Art
After the establishment of mass production techniques for CCD (charge coupled device), apparatus utilizing CCD type solid-state image pickup devices as line (linear) sensors or area image sensors are rapidly prevailing.
With the spread of portable remote terminals, MOS type solid state image pickup devices having a smaller power consumption than that of CCD type solid image pickup devices, particularly MOS type solid state image pickup devices with built-in analog/digital conversion units (hereinafter abbreviated as “A/D conversion units”) for the purposes of high performance and low cost, are under developments. An A/D conversion unit has, for example, a sample/hold circuit unit and an analog/digital converter (hereinafter abbreviated as an “A/D converter”) connected to the output of the sample/hold circuit.
A MOS type solid state image pickup device has: a number of photoelectric conversion elements (e.g., photodiodes) disposed in a matrix layout in the principal surface of a semiconductor substrate; a switching circuit unit provided for each photoelectric conversion unit; and an output signal line connecting the switching circuit units and A/D conversion unit for each photoelectric conversion element column. Each switching circuit unit includes an output transistor. This output transistor can generate an electric signal on the output signal line. The power of the electric signal represents an amount of a signal charge accumulated in a corresponding photoelectric conversion element. The output signal line is desired to have a low electric resistance and is generally made of metal material.
A built-in A/D conversion unit of a conventional MOS type solid-state image pickup device is provided per each output signal line electrically connecting to one end thereof. The electric signal generated on the output signal line by the output transistor is input to the AND converter unit. An analog voltage signal is generally input to the A/D conversion unit. The A/D conversion unit outputs a digital signal representing the input analog voltage signal, for example, to a buffer memory.
Generation of an electric signal by the output transistor is controlled, for example, in the unit of a row of photoelectric conversion elements. This control can be conducted by controlling the operation of switching circuit elements in the unit of a row of photoelectric conversion elements. To this end, one row select signal wiring line is provided for each photoelectric conversion element row.
A row select signal is supplied via the row select signal wiring line to corresponding switching circuit units to control the operations of the units.
A scan unit is formed in many cases on the same semiconductor substrate to supply the row select signal to each row select signal wiring line at a predetermined timing.
In this specification, the “photoelectric conversion element column” is intended to mean an “array of photoelectric conversion elements” disposed along an extension direction of the output signal line, among “arrays of photoelectric conversion elements” disposed in the matrix layout. The “photoelectric conversion element row” among the “arrays of photoelectric conversion elements” disposed in the matrix layout is intended to mean an “array of photoelectric conversion elements” disposed along a direction crossing the photoelectric conversion element column direction.
The operations of the scan unit, A/D conversion units, buffer memory and the like are controlled by a control unit. This control unit is formed in many cases on the same semiconductor substrate.
A MOS type solid-state image pickup device with built-in A/D conversion units has a consumption power of about ⅕ to 1/10 that of a CCD-type solid state image pickup device.
In order to improve the resolution of a solid-state image pickup device, it is preferable to form photoelectric conversion elements as many as possible at a high density. A chip size of a solid state image pickup device having a number of pixels or photoelectric conversion elements can be reduced by reducing the size of each pixel or photoelectric conversion element and forming them at a high density on one semiconductor substrate. In this case, a low cost can also be realized.
The higher the integration degree of photoelectric conversion elements on a semiconductor substrate, the pitch between photoelectric conversion elements in the photoelectric conversion element row direction becomes shorter. Namely, the pitch between photoelectric conversion element columns becomes shorter. In the MOS type solid-state image pickup device with built-in A/D conversion units, the pitch between AID conversion units becomes shorter correspondingly.
In a conventional MOS type solid-state image pickup device with built-in A/D conversion units, one output signal line is provided per each photoelectric conversion element column, and one AND conversion unit is provided per each output signal line. The sample/hold circuit unit of the A/D conversion unit has a capacitor of a relatively large electric capacitance in order to stably hold an analog voltage signal.
For example, if the integration degree of photoelectric conversion elements is set so that the pitch between photoelectric conversion elements in the photoelectric conversion element row direction becomes about 4 μm, highly sophisticated micro patterning techniques are required when AND conversion units are formed, and the manufacture cost increases.